1. Field of the Invention
The present invention relates to a method of steering a vehicle, of such a kind that the front and rear wheels are both secondarily steerable in addition to a main steer of the front wheels.
2. Description of the Prior Art
In cases where both the front and rear wheels are secondarily steerable, it is known to maintain the yaw rate and lateral acceleration unchanged irrespectively of the speed of rotation of the steering wheel by making flat the frequency characteristics of the yaw rate and lateral acceleration relative to the steering frequency through control of the respective secondary steer angles. By this, the passenger or passengers can always feel the same yaw rate and lateral acceleration and therefore can be free from an otherwise caused uneasiness at the beginning of cornering.
To this end, as described in "Gakujutsu Koen-kai Maezuri-syu No. 871017" published by Society of Automotive Engineering of Japan in May, 1987, it has been proposed to compute a front wheel secondary steer angle .delta.f and a rear wheel secondary steering angle .delta.r for the given steering angle .theta. of the steering wheel by using the following formulas (1) and (2) and thereby exercise a proportional control and derivative control over the front and rear wheels, respectively. EQU .delta.f/.theta.=Kf+Tf.multidot.S (1) EQU .delta.r/.theta.=Kr+Tr.multidot.S (2)
where Kf and Kr are proportional constants, Tf and Tr are differential constants and S is a laplace operator.
The formula (2) can be rewritten as follows. EQU .delta.r=Kr.multidot..theta.+Tr.multidot..theta..multidot.S. =Kr.multidot..theta.+Tr. (3)
In the above formulas, Kf, Kr and Tf are set to be positive whereas Tr is set to be negative. Due to this, just after the moment t1, at which the rotation of the steering wheel begins, the rear wheel secondary steer angle .delta.r exhibited by the formula (3) immediately becomes such (until the moment t2) to be of such a phase opposite to that of the front wheels and thereafter returns to the same phase as the front wheels, as is apparent from FIG. 12, in which the rear wheel secondary steer angle .delta.r is shown together with Kr.multidot..theta. and Tr.multidot..theta.. For this reason, the vehicle steering system is required to perform a complicated control and therefore the system becomes complicated, resulting in a problem of a high cost. Further, the system is required to have a high responsiveness, thus further increasing the cost.